In recent years, as a technique capable of attaining compatibility between an increase of a steel in strength, and the formation thereof into a complicated shape, a hot stamping technique has been suggested, which is a technique of pressing, at a high temperature, a steel sheet (a hot-rolled and pickled steel sheet obtained by subjecting an original steel to hot rolling and pickling; a cold-rolled steel sheet; or a galvanized steel sheet in which any one of these steel sheets is used as a base steel sheet) to produce a steel product. Hot stamping is also called, for example, hot forming or hot pressing, and is a method of heating the steel sheet to a high temperature not lower than the temperature range (Ac1 transformation point) of austenite+ferrite, and then pressing the steel sheet. According to the hot stamping, car parts each having a complicated shape can be obtained while the parts have a high strength.
Patent Literature 1 discloses a hot-rolled and pickled steel sheet or a cold-rolled steel sheet as a steel sheet usable in the hot stamping method. It is disclosed that by heightening the proportion of Si in the hot-rolled and pickled steel sheet or cold-rolled steel sheet to 0.7% or more, an improvement is made in the bonding strength of a spot-welded region thereof (the strength may be referred to as the “welding strength of the spot-welded region”; the same will be applied to the following). Patent Literature 1 also discloses that when a relationship between Ti and N is appropriately controlled and B is caused to be present in a solid solution state, the steel sheet is restrained from being deteriorated in hot formability by an increase in Si proportion.
When a car part is produced by press forming, a hot-rolled and pickled steel sheet or cold-rolled steel sheet, as described above, is used as a steel sheet to be supplied to the press forming. Besides, from the viewpoint of improving the corrosion resistance, a galvanized steel sheet obtained by galvanizing any one of these steel sheets is also used. Galvanized steel sheets are roughly classified mainly into zinc-galvanized steel sheets and Al-plated steel sheets. The zinc-galvanized steel sheets are widely used, considering the corrosion resistance, and others.
Zinc, which constitutes the galvanizing coating layer of the zinc-galvanized steel sheets, has a melting point of 419° C., and a boiling point of 907° C. to turn into a liquid phase or gas phase in a temperature range in which the steel sheet is hot-stamped. In the hot stamping step, generally, the steel sheet is heated in the atmosphere. Thus, active zinc in the state of the liquid phase or gas phase is easily oxidized so that zinc oxide (oxide film) is easily generated in the surface of the steel sheet.
A car part is obtained by hot-stamping a steel sheet, and subjecting the resultant to chemical treatment or electrodeposition coating. However, when zinc oxide (oxide film) is thinly formed onto the surface of a base steel sheet in the hot stamping step, there is caused such a problem that a coating film formed by the coating is easily peeled away (the adhesion of coating film is lowered).
As a method for avoiding this problem, Patent Literature 2 discloses the following technique: a technique of improving the adhesion of coating film by forming an oxide coating film containing Ti, Zr, Si or some other in a chemical treatment after the hot stamping. However, such a chemical treatment is unfavorable since the treatment is different from ordinary phosphate-treatments so that a liquid for the treatment needs to be changed, or other restrictions are imposed onto the producing process. Additionally, in the technique of Patent Literature 2, by use of two heating furnaces, the heating temperature in the hot stamping step is controlled, and the heating period therefor is shortened. In this way, the growth of the zinc oxide film is controlled. Although in any hot stamping step a single heating furnace is usually used, the technique referred to herein requires two heating furnaces to increase costs for facilities and in the producing process. The base steel sheet disclosed in Patent Literature 2 has a Si concentration of 0.5% or less to have a problem of not gaining a high welding strength at a spot-welded region thereof.